Study On The Petrophysical Relations Of The Typical Soils In The Huang-huai-hai Plain

The soil spatial and temporal heterogeneity increases the difficulties in makingmeasurement of soil properties and in converting scales of soil property parameters. Thereare traditional soil physical methods for measurement of soil property or state, while thereis few method for measurement of spatial heterogeneity of soil structure. In fact, soil isalso a dielectric. The electrical characteristics such as soil resistivity, electrical conductivity,dielectric constant, etc., which can be measured with relative ease in the field, are thecomprehensive reflection of some soil physical properties, making it possible to measuresoil property indirectly through soil electrical characteristic. Recently, electrical resistivitytomography (ERT), one kind of geophysical technology, obtains increasingly wideapplication in the study of soil hydrological process in vadose zone. The ERT can realizemulti-dimension, multi-scale, undisturbed, continuous monitoring of soil property or state.Petrophysical relations between soil resistivity and soil property is an importantprecondition of success application of ERT.Study on soil petrophysical relation that relies heavily on soil type and soil texturesupports application of ERT in the study of soil hydrological process in theHuang-Huai-Hai Plain. Accordingly, this study selects four typical soil (sandy loam soil ofPandian, sand soil of Daheigang, loam soil of Lizhuang, sandy clay loam soil of Chengang)to study the spectrum curve of response of the soil resistivity to measurement frequency,the sensitivity of soil resistivity to soil water content, soil texture, soil bulk density, thequantitative relationship between soil resistivity and soil water content, suction, soilhydrological property under different soil texture, soil bulk density and soil water contentconditions. The results were as follows:(1) The measurement of soil resistivity was influenced by soil water content andmeasurement frequency. In double logarithmic coordinate, resistivity of four soils shows alinear decrease along with increasing of measurement frequency. Furthermore, the increaseof soil water content tends to lower the impact of frequency fluctuation on the resistivitymeasurement. The sweep frequency curve of soil with water content above40%was moresmooth than that of dry soil. The curve of soil petrophysical relation was relatively smoothat high frequency, meaning that the sensitivity of soil resistivity to soil water contentdecreased, which is advantageous to accurate measurement of soil water content bymonitoring resistivity. (2) The sensitivity of soil resistivity to soil bulk density was related to measurementfrequency. Soil resistivity was not sensitive to small fluctuation of soil bulk density at lowfrequency (42HZ). In this case, it is difficult to determine spatial distribution of soil bulkdensity by measuring resistivity. The separation of soil petrophysical relation curve atdifferent soil bulk density increased with increasing measurement frequency, but was notvisible.(3) The soil particle of four typical soils in this region has small specific surface areaand poor adsorption property. So the main factor that influences conductivity is soilsolution, rather than soil solid component. The soil petrophysical relation curve of foursoils were very similar at low frequency and low concentration, indicating that it is difficultto discriminate their differences by measuring resistivity profile.(4) Archie empirical formula can preferably describe the relation between soilresistivity and soil water content of four soils. The prediction curve and measurementcurve was dissimilar in the relation curve of soil resistivity and soil suction. The predictioncurve and measurement curve was similar in the relation curve of soil resistivity andunsaturated hydraulic conductivity. The soil resistivity can preferably predict unsaturatedhydraulic conductivity. The trend of predictions was in accord with those of measurementsin the relation curve of soil resistivity and soil water diffusivity; however, the predictionswere slightly larger than the measurements.The above results help to improve the understanding of soil petrophysical relation inthe Huang-Huai-Hai Plain, which enables us to select the appropriate measurementfrequency and condition in using electrical resistivity tomography. It is important toachieve accurate measurement of soil resistivity or provide the reasonable explanation formonitoring result to simulate true soil property.